In the production of nitrogen derivatives of benzene, toluene, xylene, phenol, and alkyl-phenols, as in the production of explosives such as TNT (trinitrotoluene), organic compounds are treated with nitric acid. This acid is usually in the form of an aqueous solution containing nitric acid and sulfuric acid. The treatment process produces waste water containing nitric acid, nitrous acid, sulfuric acid, nitrates, nitrites and sulfates, the nitrogen-containing components being generally described below as nitro compounds. The concentration of these solute impurities is relatively low so that their removal presents a considerable difficulty.
These solutes all are thermally reducible by incineration to relatively innocuous substances, such as gaseous nitrogen, water, and carbon dioxide. As long as the combustion is carried out carefully no undesirable materials (soot) or nitrogen oxides are produced. Such a process requires the solution to be incinerated at a temperature of between 1000.degree.C and 1200.degree.C. Since the concentration of these products to be thermally reduced in the solution is relatively low an enormous amount of energy is needed to heat the effluent to this elevated temperature. A method of avoiding this has been to pass the hot dry gas produced by the incineration process through a heat exchanger having a coil through which the solution is passed, so that this solution is raised to a relatively elevated temperature before it even enters the incineration chamber. Such a method, is, however, extremely expensive due to the elevated cost of a thermal heat exchanger usable with a solution containing the various acids and impurities listed above.